Firearms are inherently dangerous devices. A controlled violent explosion happens every time the trigger is pulled. Only by careful control of propellant charges, tolerances of manufacture, and strength of materials can they be made safe to handle and use. When reloading spent cartridges there are many ways to make errors that can have negative consequences. Therefore it is extremely important to know that all of the critical parameters are within specifications. One such specification is that of the chamber size which receives the loaded cartridge. If the chamber is too small the cartridge may jam and ignite without the bolt fully closed. If the chamber is to large, explosive gasses will escape around the cartridge and be expelled toward the user. The firearms industry relies on a rigorous set of specifications for chamber dimensions and corresponding loaded ammunition. These dimensions are known as the SAAMI specifications or “SAAMI specs”. SAAMI stands for the SPORTING ARMS AND AMMUNITION MANUFACTURING INSTITUTE INC. Therefore, in theory if a firearm is manufactured with the proper size chamber all commercially available cartridges will fit in the chamber properly.
For each different firearm cartridge size, there are a set of SAAMI specifications for the firearm chamber and cartridge. While there are many dimensions that describe the size of a firearm cartridge, headspace is one that governs how the loaded cartridge fits into the chamber lengthwise when the bolt is locked into a battery or closed position. Battery refers the position of the bolt when the locking lugs of the firearm bolt or breach end of the chamber are closed and locked ready to fire. Because of manufacturing tolerances there is always some clearance in the fore/aft position of the breach locking lugs. Because the locking lugs are under spring tension from the firing pin assembly, they may not be held all the way against the aft surfaces which would define the largest closed bolt chamber dimension.
To help understand aspects of this description, the cartridge ignition process will now be discussed. A loaded cartridge is placed in the chamber of firearm. The bolt is in most cases moved forward pushing the cartridge into the chamber of a firearm such as, but not limited to, a rifle. The bolt is locked, usually by closing a handle. Once the trigger is pulled a spring urges the firing pin forward to strike the primer at the rear of the cartridge. The firing pin impact ignites the powder charge inside the cartridge. The release of the firing pin also releases spring tension on the locking lug surface holding the cartridge into the chamber. If there is any clearance or space between locking lugs of the bolt and the action retaining surfaces, the locking lugs will most likely jump rearwardly until they come to the hard stop of the action or breach. When the primer ignites the powder charge, the explosion expands inside the cartridge forcing the walls of the brass cartridge out against the walls of the firearm chamber until the cartridge can no longer expand because the cartridge is fully supported by the steel of the chamber and bolt face. In addition to radial expansion there is longitudinal expansion and movement. With no place else to go, expanding gases force the bullet out through the bore of the firearm. Before that happens, however, the expanding gases push the bullet forward towards the muzzle and push the brass case rearward against the bolt face. As the case moves rearward pushing on the bolt face and retaining lugs, it will take up any tolerances or space in the bolt lug interface with the action. This movement or jumping of the cartridge between ignition and expulsion of the bullet detracts significantly from the accuracy of a firearm. For example, if one cartridge is short and has to expand a long way before releasing the bullet, that bullet will have a different downrange point of impact than a bullet that is released from a properly sized cartridge.
In a perfect world the brass cartridge would be sized exactly to the fully expanded chamber size so that the brass cartridge does not jump or move around inside the chamber during the firing cycle. Because of manufacturing tolerances it is impossible to make every chamber of every firearm the same size. It is also impossible to make all cartridges of a particular type the same size. Due to manufacturing tolerances one must always have a slightly smaller cartridge than the chamber or else cartridges having lengths on the longer side of the tolerance will be too long to fit in the chamber. Therefore, cartridges are manufactured slightly undersized so that they will fit in even the smallest chamber size allowed by the SAAMI specs. A headspace gauge of the invention may be used, for example, for ammunition known as bottleneck cartridges most commonly used in long arms known as a rifle. The name bottleneck cartridge comes from the fact that there is a rather large case which holds the powder, and a shoulder on the case which has a frustoconical shape tapering to a cylindrical neck which holds the bullet in place at the end of the cartridge. SAAMI has determined that the best place to determine the overall length or headspace of a firearm chamber is at an imaginary datum diameter in contact with the shoulder of the cartridge. Therefore, the headspace is measured from the base of the cartridge to the datum circle on the shoulder. The reason SAAMI uses a predetermined datum diameter is that brass cartridges must have radiused edges to prevent stress cracking. Because different wall thicknesses will produce different radii it would be very difficult to determine headspace at an intersection point between walls and angled shoulders.
Those who strive for exceptional accuracy rely on custom sizing their brass cartridges to be certain that the cartridges that they make their loaded ammunition from are sized perfectly for the firearm chamber in which they will be used. Because factory made ammunition must be made to fit in every chamber, it cannot possibly have the accuracy of ammunition sized perfectly for a specific chamber. Therefore, those interested in accurate ammunition with exact tolerances must take steps to properly size the cartridges in dies during the reloading process. A gauge of this invention may, for example, be used by those who reload (shape the cartridge in a die) their own ammunition and need to know the exact true headspace of the firearm chamber in which they will make the most accurate cartridge case for that particular chamber.
As used herein, the term “true headspace” or “actual headspace” refers to the dimension measured from a closed chamber when all parts which define the chamber dimension have been expanded to their physical limits. Currently, there is no practical way to tell what the “true headspace” dimension of a chamber is because the chamber is inaccessible (other than through the bore of the firearm) when the bolt is closed. This is a very important distinction because the current practices to attempt to obtain this dimension are either very difficult to perform or give inferred measurements. The most common way to approximate the headspace is by measuring brass cases which have been fired in the chamber in question. This method has drawbacks, and most people are unaware that these drawbacks obtain incorrect results.
Currently, most reloaders measure a fired brass case to determine headspace. This practice is inaccurate for the following reasons. When a brass cartridge is fired the brass case expands radially outward to the chamber wall of the firearm and then retracts a bit due to the elastic limit the brass. The amount it retracts is related to the elastic limit and hardness of the brass. If a brass case has been fired several times it will work harden and spring inwardly away from the chamber wall more than a cartridge that is annealed prior to firing. Excessive explosive forces from loading high pressure or magnum loads can also stretch the brass and make it too long for the chamber. To make reloaded brass fit the chamber properly so that the bolt can be closed easily every time, it needs to be forced into a shape forming die to restore its shape to fit the chamber every time. Those interested in accuracy will anneal their cartridges with a carefully applied flame heat. This allows the brass cartridge to be resized during the reloading process by forcing it into a reloading die to shape the cartridge to the correct headspace. The problem is very few people will anneal each brass case prior to reloading. For example, an annealed case may only shrink back one or two thousands of an inch from the chamber. A cartridge that has been fired multiple times and is work hardened may shrink back 0.006 inch. For users with several brass cases, and various numbers of firings among the lot, it would be very hard to tell what the headspace is by measuring fired brass. This is one of the problems the invention solves by measuring the chamber and not the brass.
Most people who reload will have brass with different numbers of firings on the brass. This sets up an additional problem where amongst the population of brass cases which are going to be resized there can be many different brass hardness levels amongst the case population. This results in cases which come out of the die at different dimensions solely based on the brass hardness. The only way to get all the brass to be the same dimension is to anneal the cases prior to resizing. Therefore, the custom made properly sized cartridge which has been annealed and reshaped in a die will fit precisely in the chamber.
One current method of approximating headspace is to purchase a set of so-called Go, No-Go gauge pins. One gauge is too long and the other is too short for the specific chamber size of the firearm. In order to determine if the chamber is in specification one installs the Go gauge and makes certain that the bolt can be closed on that gauge. If the bolt closes, the chamber is long enough to accept the longest SAAMI spec ammunition cartridge. One then installs the No-Go gauge and the bolt should not be able to be closed. If the bolt closes, the chamber is too long. However, one never really knows what the dimension of the chamber is between the Go and No-Go gauge. One only knows that it is within the short and long limits. One can make a metal cast of the chamber with a special fixturing metal. The metal melts at a low temperature (˜160° F.) and can be poured into the chamber, allowed to cool and then removed. The problem here is that half of the headspace dimension is in the bolt face, the other half is in the barrel chamber. And there is no way to take into account the bolt lug clearance. So if you extract the cast of the portion of the chamber in the barrel you still need to figure out how much is in the bolt face and where the interface is between the chamber and bolt recess. This method also does not push the bolt locking lugs into their battery position against the aforementioned spring tension. On the other hand, using the brass measurement technique one encounters a whole new set of problems. Brass is a material which work-hardens. That means that as the material is stressed it changes properties. One of those properties is ductility. Therefore, if a brass cartridge is fired many times it will be harder and less ductile than one which is fired from an annealed state. This fact is lost on most people who measure brass cartridges to determine headspace in the chamber. Tools are currently sold which attach to off-the-shelf vernier calipers to measure headspace from a fired brass cartridge. To determine the headspace on a fired brass case one attaches a cylinder with a known inside diameter equal to the datum diameter used to measure the headspace.
Based on the summary of the procedure provided above, it will be appreciated that there is a need for a device that measures the true headspace of a chamber when the bolt is closed.